Avoiding Plastic Injection Molding Defects: Key Preventive Measures

Plastic injection molding is one of the most common production processes here in China. And most Chinese manufacturers are not very good at setting up and maintaining their presses and their molds…

If the right polymer is used, and if the factory adopts the right process control measures, there is a low probability for things to go wrong. I have met many manufacturers that don’t understand that…

There are a multitude reasons for parts to be rejected. Some of the more common defects are listed below, along with the key preventative measures.

Sink Marks

A sink mark is a depression on the surface of the molded part and is generally found where there are bosses or ribs on the underside of the molding. These intersections result in a thicker material section and the cooling is uneven in these areas.

As the material cools, the outer surface cools faster than the inner and with the outer surface solidified, the inner molten material begins to cool, solidify and shrink thus pulling on the already solidified surface.

This irregular cooling and shrinking of the material around thicker areas is the cause of most sink marks.

Key Preventative Measures

Ensure wall thicknesses are uniform wherever possible

Make ribs 50% to 80% of the wall thickness it is connected to

Select the thickest section of the part to add the injection gate, this allows the thickest parts to injected first

Do not have the gate size too small as this will prevent the optimum fill and packing of the mold

Avoid large thick areas in the design, better to have a ribbed or honeycombed design if you are looking for strength and rigidity

Increase the injection pressure during the process, this will force more material into the mold and reduce the shrinkage

Reduce the injection temperature of the material and the mold

Voids

Voids or air pockets tend to occur in thicker sections of a molded part, this is caused when the surface cools and solidifies faster than the inner material.

As the inner material is cooling it also shrinks, but if it is unable to fully shrink, the material molecules will be pulled apart thus creating voids.

Ensure material is dried correctly, moisture in the material will cause voids

Weld Lines

Weld lines are created when two melt fronts come together but do not bond with each other because they have partially solidified. When the part is fully solidified, the weld lines often look like two plastic planes have come together.

Key Preventative Measures

Having a design where the flow is continuous and not split will prevent weld lines

If a split in molten plastic is unavoidable, ensure the position of the gate allows the melt fronts to come together evenly at the same time

Raising the temperature of the mold or molten plastic will help when the two melt fronts come together

Increasing the injection speed will help the melt fronts come together quicker, while still molten

Short Shots

Short shots occur because the mold is not completely full before the plastic solidifies, resulting in a partially molded part.

Another reason for short shots is trapped air or gas in the mold cavity, the molten plastic cannot occupy the same space as the gas, therefore resulting in a short shot mold.

Key Preventative Measures

Ensure the gate size is large enough to fill the mold cavity

Ensure the tool has adequate gas/air vents to allow the gas build up to escape the cavity

Increase the temperature of the mold and plastic being injected, this will help fill the mold faster

Make sure there is enough material being injected into the mold per shot

Increase the injection pressure

Burn Marks

Burn marks show up on the finished part as charred or dark discoloration which is caused by trapped gas overheating in the mold.

It can also be an effect of plastic degradation due to excessive heat or by the injection speed being too fast.

Key Preventative Measures

Ensure the tool has adequate gas/air vents to allow the gas build up to escape the cavity

Reduce the injection speed

Reduce the injection pressure

Reduce the temperature of the plastic during injection

Make sure the part design does not have any feather edges as this can increase the chances of burn marks

Flash

Flash is a defect where molten plastic has escaped the cavity, generally at tool parting lines or vent areas or even down ejector pins.

Key Preventative Measures

Ensure the clamping pressure is being maintained during the injection process

Make sure the surfaces of the mold is clean and not damaged before it is loaded into the machine

Reduce the injection pressure

Reduce the temperature of the mold and plastic being injected

Check the diameter of the ejector pins and the pin hole tolerances, if the tolerance is too wide this could allow molten plastic to flow down between the pin and hole surface

Make sure the venting is not too large

Flow Lines

Flow lines or streaks in the surface of the finished part can be a cause for reject if the part is required for aesthetics. Flow lines are caused by the molten plastic flowing at different speeds throughout the cavity and the change in direction of the melt front due to the design of the mold.

These lines occur as the plastic starts to solidify at different speeds, in some cases the molecules are dragged along the surface of the mold causing streaks.

Key Preventative Measures

Ensure the design of the tool has rounded edges wherever possible.

Increase the speed and pressure of the injection process

Increase the temperature of the mold and the molten plastic during the injection process

Increase the size of the gate to allow optimum mold filling

Warping

Warping occurs when the part cools unevenly and creates internal tension within the part. When the part is ejected from the tool, the part will warp as the internal tension is released.

Key Preventative Measures

Ensure the cooling time is controlled to allow the plastic to solidify evenly, the variables requiring control would be cooling time and the cooling temperature rate

Reduce the temperature of the material during injection and the temperature of the mold

Make sure the gate position allows the cavity to fill evenly as possible, if the cavity is filled in all extreme points the same, the solidification process is more even which helps eliminate warping of the part

Select plastic materials that are less likely to shrink and deform. Semi-crystalline materials such as Polyphthalamide (PPA), Polyetherketone (PEK), and High Temperature Nylon (HTN) are generally more prone to warping.

Part sticking in tool

When the molded part sticks to the mold, it fails to separate from the cavity or is held by a feature of the tool design which can occur for many reasons.

Key Preventative Measures

Ensure the ejector pins are not broken, without ejector pins the part cannot be pushed out of the mold

If the draft angle is too shallow, there will be too much surface friction between the cavity surface and the part preventing it from being ejected. In this case, draft angle needs to be increased

Surface finish to too rough causing too much surface friction between the cavity surface and the part preventing it from being ejected. Two things to change in this instance, smooth and polish the surface of the cavity if a smooth surface is permissible in the design, if not, the draft angle needs to be increased to allow for the rough surface to separate between the part and the cavity surface

Ensure there are no undercuts in the design, these would prevent the part being ejected as the part would hang on the undercut surface

Make sure there are no defects or scratches on the cavity surface as these could act as undercuts and hold the part during ejection

Ensure a release agent is applied to both parts of the tool between each injection cycle

Quality Inspections & Supply Chain Management in China

This blog is written by Renaud Anjoran, an ASQ Certified Quality Engineer who has been involved in Chinese manufacturing since 2005.
Renaud's company performs factory audits/evaluations, product QC inspections, and factory process improvements.